This invention relates generally to electrical interconnections for connecting printed circuit boards.
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling more data at higher speeds than connectors of even a few years ago.
As signal frequencies increase, a greater possibility exists for electrical noise to be generated in the connector in forms such as reflections, cross-talk and electromagnetic radiation. Therefore, electrical connectors are designed to control cross-talk between different signal paths, and to control the electrical properties of each signal path. In order to reduce signal reflections in a conventional connector module, the impedance of each signal path is controlled to avoid abrupt changes of impedance that can cause signal reflections. The impedance of a signal path is generally controlled by varying the distance between a conductor carrying the signal path and adjacent conductors, the cross-sectional dimensions of the signal conductor, and the effective dielectric constant of materials surrounding the signal conductor.
Cross-talk between distinct signal paths can be controlled through the use of shielding. Signal paths may be arranged so that they are spaced further from each other and nearer to a shield, which may be implemented as a grounded metal plate. The signal paths tend to electromagnetically couple more to the ground conductor, and less with each other. For a given level of cross-talk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors are maintained.
Electrical connectors can be designed for single-ended signals as well as for differential signals. A single-ended signal is carried on a single signal conducting path, with the voltage relative to a common ground reference being the signal. For this reason, single-ended signal paths are sensitive to any electromagnetic radiation that may couple to signal conductors.
To avoid this sensitivity, signals, particularly low voltage signals, may be communicated differentially. Differential signals are signals represented by a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducing paths of a differential pair are arranged to run near each other. If a source of electrical noise is electromagnetically coupled to the differential pair, the effect on each conducting path of the pair should be similar. Because the signal on the differential pair is treated as the difference between the voltages on the two conducting paths, a common noise voltage that is coupled to both conducting paths in the differential pair does not affect the signal. As a result, a differential pair is less sensitive to cross-talk noise, as compared with a single-ended signal path.
Examples of differential electrical connectors are shown in U.S. Pat. Nos. 6,293,827, 6,503,103, 6,776,659, and 7,163,421, all of which are assigned to the assignee of the present application and are hereby incorporated by reference in their entireties.
While electrical connector designs have provided generally satisfactory performance, the inventors of the present invention have noted that at high speeds (for example, signal frequency of 3 GHz or greater), presently available electrical connector designs may not sufficiently provide desired cross-talk, impedance and attenuation mismatch characteristics, particularly for very dense connectors,